BACKGROUND: Pleuropulmonary blastoma is a rare, aggressive neoplasm that typically occurs in young children. It has been classified as type I, II, or III on the basis of the cystic versus solid nature of the lesion as well as the histologic appearance. Although it has been speculated that type I lesions may have a tendency to progress into type III lesions, no such case has been reported to date. METHODS: A case of type I pleuropulmonary blastoma in a girl age 2 years 9 months was found in our departmental archive. This case, originally diagnosed as a hamartoma over 20 years ago, predated the description of this pathologic entity. Over a 3-year period, the patient underwent excisions of a primary tumor and 3 subsequent recurrences, thus allowing us to follow the natural history of this neoplasm. RESULTS: The primary tumor was a large, multicystic mass (roughly 90% cystic by volume) with benign histologic appearance except for occasional foci with bland, embryonal rhabdomyosarcomatous features. In subsequent recurrences, the resected specimens became increasingly solid and had an anaplastic, multiphasic mesenchymal pattern. CONCLUSIONS: The course of the patient described here represents the first case in which transition from type I (cystic) to type III (solid) was documented.
